1. Field of the Invention
The present invention relates to a fluid-filled power unit mount through which a power unit of a vehicle is mounted on a vehicle body, and more particularly to such a power unit mount of the type wherein an elastic support member is securely disposed between inner and outer cylinderical members and formed with a chamber filled with a fluid for damping vibration.
2. Description of the Prior Art
In general, a power unit constituted of an engine and a transmission is supported through power unit mounts (engine mounts) on a vehicle body in order to prevent engine vibration and the like from being transmitted to the vehicle body. Such a power unit mount usually includes an elastic support member made of rubber or the like and securely put between two attachment members which are respectively connected to the vehicle body and the power unit. However, with such a power unit mount, there is possibility of the elastic member being broken and cut into two by any cause. In order to overcome this difficulty and to make the power unit mount small-sized, a power unit mount of so-called inner-outer cylinder type has been proposed and put into practical use in which an elastic support member is securely disposed between inner and outer cylinders. The inner cylinder is connected to one of a power unit and a vehicle body, while the outer cylinder is connected to the other of them. With the thus configurated power unit mount, the load of the power unit is supported by the elastic member, and the inner cylinder is prevented from getting out of the outer cylinder even if the elastic member is broken and cut into two because the inner cylinder is surrounded by the outer cylinder.
Such an inner-outer cylinder type power unit mount is disclosed, for example, in Japanese Patent Provisional Publication No. 61-65935. In this power unit mount, the elastic support member between the inner and outer cylinders is formed with two fluid chambers which are located opposite with respect to the inner cylinder and communicated with each other through an orifice formed in a block securely mounted on the inner cylinder. Accordingly, when relative displacement between the inner and outer cylinders is made along with vibration of the power unit, the elastic support member deforms so that one fluid chamber expands simultaneously with contraction of the other fluid chamber, thereby causing the fluid within the fluid chambers to move through the orifice. This damps vibration of the power unit.
However, in such a power unit mount, the respective fluid chambers are formed in the elastic support member such that the elastic support member itself serves as side wall of the fluid chambers. Additionally, the elastic support member is required to support a larger load of the power unit and therefore necessarily has a larger spring constant, thereby unavoidably raising the spring constant due to expansion elasticity of the fluid chambers. Consequently, vibration frequencies to be damped by the vibration damping mechanism constituted by the fluid chambers and the orifice unavoidably fall within a relatively high frequency region which is far from a relatively low frequency region such as of engine shake and/or idling vibration. Thus, it is impossible to effectively suppress engine shake and idling vibration, and therefore low frequency vibration is still allowed to be input to the vehicle body, thereby deteriorating ride-on feeling of the vehicle. Furthermore, because of the orifice being formed in the block securely attached to the block, increasing the opening sectional area of the orifice is restricted thereby to minimize the orifice sectional area. Therefore, there is possibility of the liquid in the orifice sticking during high frequency vibration. As a result, vibration damping in a high frequency region also cannot be effectively achieved.